The FBI is urging state election officials to beef up their computer systems' security in light of two cyberattacks this summer.

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The FBI has found evidence that two state election databases were infiltrated this summer by foreign hackers, according to a Yahoo News report Monday.

That's led the the agency to urge state election officials throughout the US to strengthen their computer systems' security, the report said.

The bureau's cyber division issued the warning on August 18 in a "flash" alert titled "Targeting Activity Against State Board of Election Systems" (PDF). The alert said "the bureau was investigating cyberintrusions against two state election websites this summer, including one that resulted in the 'exfiltration,' or theft, of voter registration data," according to Yahoo News, which obtained a copy of the alert.

The warning didn't name the states but sources told Yahoo voter registration databases in Arizona and Illinois were targeted. In Illinois, hackers stole the personal data of up to 200,000 of the state's voters. In Arizona's case, malicious software was found in the system but no data was taken, a state official told Yahoo News.

The bureau suggested the two attacks may be linked but did not name the country where they may have originated, the report said.

The FBI declined to comment on the specific alert. "The FBI routinely advises private industry of various cyber threat indicators observed during the course of our investigations. This data is provided in order to help systems administrators guard against the actions of persistent cyber criminals."

Earlier this month at a press event in Washington, D.C., Homeland Security Secretary Jeh Johnson said the government is concerned cyberattackers could disrupt the November presidential election. He said the government should consider whether elections should be treated as "critical infrastructure."